Summary/Abstract In the event of a large scale radiologic or nuclear event, the immediate needs are to discriminate exposed individuals from the worried well and identify individuals at risk for radiation-induced tissue damage. Humans vary in their sensitivity to radiation-induced tissue injury, the factors that control radiation sensitivity are not fully understood, and there are no biomarkers to identify individuals at risk for radiation-induced tissue damage. The skin is an immunologically active barrier tissue that will be impacted by all types of external radiation events. Skin is visually observable, accessible to non-invasive testing and easily sampled, making it amenable to a variety of testing modalities. We propose to irradiate and study living, immunologically intact human skin grafts on immunodeficient NSG mice to identify biomarkers that detect exposure to radiation and predict radiation induced tissue damage. In Aim 1, we will study the effects of irradiation on adult human skin grafts carried by NSG mice. We will study DNA damage repair proteins, reactive oxygen species, epithelial barrier function and gene expression changes in adult human skin grafts exposed to 0, 0.5, 1.0, 2.0 or 5.0 Gy irradiation. We will identify biomarkers that detect radiation exposure within 24 hours of exposure (Phase I), remain detectable longer term and/or correlate with later tissue injury (Phase II) and we will validate these biomarkers in a separate validation cohort of 30 human skin donors (Phase III). In Aim 2, we will study the effects of irradiation on human neonatal foreskin grafts, a tissue that contains antigen presenting cells but lacks T cells, as a model for both pediatric and immunocompromised populations. We will test biomarkers identified in Aim 1 for their ability to detect radiation exposure within 24 hours and to predict tissue injury in neonatal foreskin. If necessary, we will identify new biomarkers for use in pediatric and immunocompromised populations. In Aim 3, we will develop rapid point-of-care tests to detect the biomarkers we identify in Aims 1 and 2. The skin is an accessible tissue that can be studied as a proxy to predict the risk of radiation-induced injury at other tissue sites. By studying immunologically intact skin from a diverse population of human donors, we will identify biomarkers that are useful in i) humans, ii) a diverse, outbred population with differing sensitivities to radiation, and iii) pediatric and immunocompromised populations. Identification of biomarkers that predict subsequent tissue damage in skin will identify radiation sensitive individuals. Our point-of-care tests have the potential to rapidly screen potentially exposed populations and identify individuals who are at risk for tissue damage and will require further medical attention.